Radiopharmaceuticals are being designed, synthesized and tested for their ability to localize in specific tissues and to yield information about metabolic pathways. For example, radiolabeled analogues of metabolites will be studied, as well as active site directed gamma ray labeled enzyme inhibitors. Such compounds may permit distinction between normal and abnormal tissues by rate of uptake or discharge quantity accumulated. Procedures such as the attempted displacement of the radiolabel from a site, by means of a more raadily bound enzyme inhibitor, will be tried in order to enhance the ability to detect localized abnormalities during life.